[PATCH] commands: sync: add new command to flush cached writes

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

In barebox, the dirty block cache for a given device is flushed when
closing all file descriptors to it, a cached block needs to be evicted
due to pressure or when barebox shuts down.

There are valid uses to do it explicitly however:

  - When benchmarking write speed to a block device
  - Before forced resets, because a regular reset would run shutdown
    code that should be omitted.

Add a sync command that does this. Unlike the userspace variant, we
don't both to implement a argument-less sync that's global. Users are
supposed to flush the files they are actually interested in, e.g.:

  sync /dev/mmc2.0

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 commands/Kconfig  | 14 ++++++++++++++
 commands/Makefile |  1 +
 commands/sync.c   | 42 ++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 57 insertions(+)
 create mode 100644 commands/sync.c

diff --git a/commands/Kconfig b/commands/Kconfig
index 5b512f1bbac7..a8b7037618cc 100644
--- a/commands/Kconfig
+++ b/commands/Kconfig
@@ -1871,6 +1871,20 @@ config CMD_DETECT
 		  -e	bail out if one device fails to detect
 		  -a	detect all devices
 
+config CMD_SYNC
+	tristate
+	prompt "sync"
+	help
+	  sync - flush cached writes
+
+	  Usage: sync DEVICE
+
+	  Synchronize cached writes to persistent storage of DEVICE
+	  immediately instead of waiting for block device to be closed,
+	  cache eviction or the regular barebox exit.
+
+	  This command is mainly useful for benchmarking.
+
 config CMD_FLASH
 	tristate
 	prompt "erase, protect and unprotect"
diff --git a/commands/Makefile b/commands/Makefile
index 4ca7ba7eb609..a9dbead4389f 100644
--- a/commands/Makefile
+++ b/commands/Makefile
@@ -41,6 +41,7 @@ obj-$(CONFIG_CMD_REGINFO)	+= reginfo.o
 obj-$(CONFIG_CMD_CRC)		+= crc.o
 obj-$(CONFIG_CMD_CLEAR)		+= clear.o
 obj-$(CONFIG_CMD_TEST)		+= test.o
+obj-$(CONFIG_CMD_SYNC)		+= sync.o
 obj-$(CONFIG_CMD_FLASH)		+= flash.o
 obj-$(CONFIG_CMD_MEMINFO)	+= meminfo.o
 obj-$(CONFIG_CMD_TIMEOUT)	+= timeout.o
diff --git a/commands/sync.c b/commands/sync.c
new file mode 100644
index 000000000000..6add67cd0876
--- /dev/null
+++ b/commands/sync.c
@@ -0,0 +1,42 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+/* sync - flushing support */
+
+#include <common.h>
+#include <command.h>
+#include <fcntl.h>
+#include <fs.h>
+
+static int do_sync(int argc, char *argv[])
+{
+	int ret, fd;
+
+	if (argc != 2)
+		return COMMAND_ERROR_USAGE;
+
+	fd = open(argv[1], O_WRONLY);
+	if (fd < 0) {
+		printf("open %s: %m\n", argv[1]);
+		return 1;
+	}
+
+	ret = flush(fd);
+
+	close(fd);
+
+	return ret;
+}
+
+BAREBOX_CMD_HELP_START(sync)
+BAREBOX_CMD_HELP_TEXT("Synchronize cached writes to persistent storage of DEVICE")
+BAREBOX_CMD_HELP_TEXT("immediately instead of waiting for block device to be closed,")
+BAREBOX_CMD_HELP_TEXT("cache eviction or the regular barebox exit.")
+BAREBOX_CMD_HELP_END
+
+BAREBOX_CMD_START(sync)
+	.cmd		= do_sync,
+	BAREBOX_CMD_DESC("flush cached writes")
+	BAREBOX_CMD_OPTS("DEVICE")
+	BAREBOX_CMD_GROUP(CMD_GRP_HWMANIP)
+	BAREBOX_CMD_HELP(cmd_sync_help)
+BAREBOX_CMD_END
-- 
2.39.2